The U.S. Historical Climatology Network (U.S. HCN) was compiled
in response to the need for an accurate, unbiased, modern historical
climate record for climate change research. The Carbon Dioxide Research
Program of the U. S. Department of Energy and the
National Climatic Data Center (NCDC)
of the National Oceanic and Atmospheric Administration (NOAA)
established a network of 1219 stations in the contiguous United States for the specific purpose of
compiling a data set suitable for detecting and monitoring climate change over the past two
centuries. This network, known as the U.S. Historical Climatology Network (U.S. HCN), and the resulting
data set were initially documented by Quinlan et al. (1987) and made available free of charge
through the Carbon Dioxide Information Analysis Center (CDIAC). The data presented in Quinlan
et al. (1987) extended only through 1984. In 1990 NCDC and CDIAC revised and updated the
HCN data records through 1987 (Karl et al. 1990). In addition, using the techniques of Karl et al.
(1988), NCDC generated temperature files in which the biases introduced by urbanization effects
were removed.

The new revision 3 (Easterling et al. 1996) data represent the best available data from the United States for analyzing
long-term climate trends on a regional scale and may be used for studies attempting to determine
the climatic impacts of increased concentrations of greenhouse gases. The data for most stations
extend through December, 1994, and a majority of the station records are complete for at least 80
years. Unlike many data sets that have been used in past climate studies, these data have been
adjusted to remove biases introduced by station moves, instrument changes, time-of-observation
differences, and urbanization effects.

At this revision (Revision 3) the data set has been extended through the end of 1994 and several
stations have been added (54) and deleted (52). Twenty-five of these additions/deletions involved
stations who's official station name and number changed, but who did not actually move.
These revisions have resulted in a network with 1221 stations (Figure 1).

One of the objectives in establishing the U.S. HCN was to detect secular changes of regional
rather than local climate. Therefore, only those stations that were not believed to be influenced to
any substantial degree by artificial changes of local environments were included in the network.
Some of the stations in the U.S. HCN are first order weather stations, but the majority were selected
from U.S. cooperative weather stations (approximately 5000 in the United States). To be included
in the U.S. HCN, a station had to be active (in 1987), have at least 80 years of mean monthly
temperature and total monthly precipitation data, and have experienced few station changes. An additional criterion that was
used in selecting the 1221 U.S. HCN stations, which sometimes compromised the preceding criteria,
was the desire to have a uniform distribution of stations across the United States.

The 1221 station U.S. HCN database contains station histories, monthly temperature (maximum,
minimum, and mean) data, and total monthly precipitation data that were compiled by NCDC after
being extracted from digital and nondigital data sets archived at NCDC. These data sets originated
from a variety of sources, including climatological publications, universities, federal agencies,
individuals, and data archives. All stations were quality controlled by NCDC with the use of
outlier and areal edits, and each station in the network was corrected for time-of-observation
differences, instrument changes, instrument moves, station relocations, and urbanization effects
(Karl et al. 1986; Karl and Williams 1987; and Karl et al. 1988). A unique feature of the data set
is that, within most temperature and precipitation data files, both original and adjusted estimates
are given, along with confidence factors for each adjusted estimate. Another unique feature of the
database is that in relation to the long periods of record, a small portion of the data are represented
as missing. In order to make the U.S. HCN record as serially complete as possible, missing data have
been estimated by using data from neighboring stations. The majority of the
1221 stations have had data records that are serially complete since 1900; where serially complete
is defined as having original or adjusted data available for all months after the reported serially
complete date for a given station.

The U.S. HCN database represents the best monthly temperature and precipitation data
set available for the contiguous United States. It provides an accurate, serially complete, modern
historical climate record that is suitable for detecting and monitoring longterm climatic changes
on a regional scale and may be used for studies attempting to determine the climatic impacts of
increased concentrations of greenhouse gases. The U.S. HCN climate record may also be used by
dendrochronologists and paleoclimatologists for calibrating tree ring growth, pollen, and marine
plankton data or by those studying the climatic impacts of periodic events such as El
Nino/Southern Oscillation or volcanic eruptions. Those studying longterm climatic changes on
smaller scales, may want to review the information given in the appendices in order to identify the
stations most suitable for their research needs.